In general, stainless steels are more difficult to machine than carbon and low-alloy steels because stainless steels have high strength and work-hardening rates compared to the carbon and low alloy steels. Consequently, it is necessary to use higher powered machines and lower machining speeds for machining the known stainless steels than for machining carbon and low-alloy steels. In addition, the useful life of a machining tool is often shortened when working with the known stainless steels.
AISI Types 304L, 316L, 321 and 347 stainless steels are austenitic, chromium-nickel and chromium-nickel-molybdenum stainless steels having the following compositions in weight percent:
______________________________________ Type 304 L Type 316 L Type 321 Type 347 wt. % wt. % wt. % wt. % ______________________________________ C 0.03 max 0.03 max 0.08 max 0.08 max Mn 2.00 max 2.00 max 2.00 max 2.00 max Si 1.00 max 1.00 max 1.00 max 1.00 max P 0.045 max 0.045 max 0.045 max 0.045 max S 0.03 max 0.03 max 0.03 max 0.03 max Cr 18.0-20.0 16.0-18.0 17.0-19.0 17.0-19.0 Ni 8.0-12.0 10.-14.0 9.0-12.0 9.0-13.0 N 0.10 max 0.10 max 0.10 max -- Mo -- 2.0-3.0 -- -- Ti -- -- 5 .times. (% C + -- % N) to 0.70 Nb + Ta -- -- -- 10 .times. % C to 1.10 Fe Bal. Bal. Bal. Bal. ______________________________________
Source: METALS HANDBOOK.RTM. Desk Edition; Chapt. 15, pages 2-3; (1985). The AMS standards for these alloys restrict copper to not more than 0.75%.
The above-listed chromium-nickel and chromium-nickel-molybdenum stainless steels are known to be useful for applications which require good non-magnetic behavior, in combination with good corrosion resistance. In order to overcome the difficulties in machining the known stainless steels, some grades of stainless steels have been modified by the addition of elements such as sulphur, manganese, or phosphorus and/or by maintaining carbon and nitrogen at very low levels. However, there continues to be a demand for improved machinability in chromium-nickel and chromium-nickel-molybdenum stainless steels, particularly for production-type machining operations such as on an automatic screw machine.
Given the foregoing, it would be highly desirable to have an austenitic stainless steel that provides better machinability than is provided by the known austenitic stainless steels.